In a known connection structure, an output shaft of a reducer is connected with one end side of a hollow link member having a rectangular cross section shape and including first and second walls facing each other and a pair of connecting walls connecting the first and second walls (refer to Japanese Unexamined Patent Application, Publication No. 2006-167863, for example). In this structure, the output shaft of the reducer is connected with the link member so that the output shaft of the reducer has surface contact with an outer surface of the first wall of the link member. In this connection structure, a space inside the hollow link member is used for disposition of pipes and wires.
The use of the inside of the hollow link member for disposition of pipes and wires is also disclosed in other known literatures (refer to Japanese Unexamined Patent Application, Publication No. H07-328982, for example).
A known connecting member that connects an arm with an output shaft of an actuator such as a motor includes a plate-like primary member fixed to the output shaft of the actuator, and a ring-shaped auxiliary member connected to the periphery of a hole provided at an end part of the primary member (refer to Japanese Unexamined Patent Application, Publication No. 2014-226738, for example).